5. SPIRALS AND IRREGULARS

Spiral galaxies usually have extended, low-density stellar halos
(Zibetti et
al. 2004)
whose density decreases with
~ r-3. In the Milky Way the stellar halo consists of old,
metal-poor stars and globular clusters on eccentric prograde or
retrograde orbits (see
Freeman &
Bland-Hawthorn 2002
and
Helmi 2008
for recent reviews of the Galactic halo). As many of half of the field
stars in the halo may have originated in disrupted globular clusters
(Martell &
Grebel 2010;
see
Odenkirchen et
al. 2001a
for an example).

CDM simulations
suggest that in galaxies with few recent mergers the fraction of halo
stars formed in situ amounts to 20% to 50%
(Zolotov et
al. 2009).
Johnston et
al. (2008)
propose that halos dominated by very early accretion show higher
[/Fe] ratios, whereas
those that accreted mainly high-luminosity satellites should exhibit
higher [Fe/H].

Early-type spirals show prominent bulges, which become less pronounced
and ultimately vanish in late-type spirals and irregulars. Classical
bulges (found in early-type to Sbc spirals) resemble elliptical
galaxies in their properties, are dominated by old, mainly metal-rich
stars with a large metallicity spread, show hot stellar kinematics,
and follow a de Vaucouleurs surface brightness profile just like
typical elliptical galaxies. Pseudobulges (in disk galaxies later
than Sbc) resemble disk galaxies, have similar exponential profiles,
are rotation-dominated, and may contain a nuclear bar, ring, or
spiral. They are believed to form from disk material via secular
evolution
(Kormendy &
Kennicutt 2004).
A recent
analysis combining data from SST and GALEX found that all bulges show
some amount of ongoing star formation, regardless of their type
(Fisher et
al. 2009),
with small bulges having formed 10 to 30% of their mass in the past 1 to
2 Gyr
(Thomas & Davies
2006).
Extracting a sample of > 3000 nearby edge-on disk galaxies from the SDSS,
Kautsch et
al. (2006)
showed that approximately 30% of the edge-one galaxies are bulge-less
disks.

Noguchi (1999)
suggested that massive clumps forming
at early times in galactic disks move towards the galactic center due
to dynamical friction, merge, and form the galactic bulge. This
scenario leads to the observed trend of increased bulge-to-disk ratios
with increased total galactic masses.
van den Bergh
(2002)
noted that while most of the galaxies observed
in the Hubble Deep Fields at z < 1 have disk-like
morphologies, most
galaxies at z > 2 look clumpy or chaotic. Analyzing such "clump
clusters" and "chain galaxies" in the Hubble Ultra Deep Field,
Elmegreen et
al. (2009)
find that the masses of
the star-forming clumps are of the order to 107 to 108
M.
Bournaud et
al. (2007)
argue that clusters of such massive, kpc-sized clumps can form bulges in
less than 1 Gyr, while the system as a whole evolves from a violently
unstable disk into a regular spiral with an exponential or double
exponential disk profile on a similarly rapid time scale. While the
coalescence of these clumps resembles a major merger with respect to
orbital mixing, the resulting bulge has no specific dark-matter
component, which distinguishes it from bulges formed via galaxy mergers
(Elmegreen et
al. 2008).

Disks are the primary sites of present-day star formation in spiral
galaxies, and it seems likely that they have continued to form stars
for a Hubble time. Disks show ordered rotation, and their stars move
around the galactic center on near-circular orbits. The rotational
velocities greatly exceed the velocity dispersion by factors of 20
or more.

Gas-deficient early-type disk galaxies show little activity at the
present time, while gas-rich late-type disks experience wide-spread,
active star formation. Star formation occurs mainly in the midplane
of the thin disks, in particular along spiral arms, where recent
events are impressively traced by giant H II regions. Spiral
density waves may induce star formation (see
Martínez-García et al. 2009;
and references therein), although it has been suggested that this
mechanism may contribute less than 50% to the overall star formation rate
(Elmegreen &
Elmegreen 1986).

In the Milky Way, the star formation in the disk was not constant, but
shows extended episodes of increased and reduced activity (e.g.,
Rocha-Pinto et
al. 2000),
a radial metallicity gradient, a G-dwarf problem, and a large
metallicity scatter at all ages
(Nordström et
al. 2004).
The thin disk
is embedded in a lower-density, kinematically hotter stellar
population consisting of older, more metal-poor stars - the thick disk
(Gilmore & Reid
1983;
Bensby et al. 2005).
The chemical similarity of Galactic bulge and thick disk stars might
suggest that the Milky Way does not have a classical bulge
(Meléndez et
al. 2008).

Dalcanton &
Bernstein (2002)
showed that thick
disks are ubiquitous also in bulge-less late-type disk galaxies, which
indicates that their formation is a universal property of disk
formation independent from the formation of a bulge. A variety of
mechanisms for the formation of thick disks has been proposed,
including formation from accreted satellites, gas-rich mergers,
heating of an early thin disk by mergers, heating via star formation
processes, and radial migration (e.g.,
Wyse et al. 2006;
Brook et al. 2004;
Kroupa 2002;
Bournaud et
al. 2009;
Roskar et al.2008).
Sales et al. (2009)
suggest that the eccentricity distribution of thick disk stars may
permit one to distinguish between these scenarios.

As noted by
van den Bergh
(2002)
based on an analysis of the Hubble Deep Fields, roughly one third of the
objects at z > 2 seem to be experiencing mergers. He suggests
that from 1 <
z < 2 a transition from merger-dominated to disk-dominated star
formation occurred. Moreover, he finds that at z > 0.5, there are
fewer and fewer barred spirals. While early-type galaxies assume
their customary morphologies relatively early on, 46% of the spirals
at 0.6 < z < 0.8 are still peculiar, and with higher
redshift, the spiral arm patterns become increasingly chaotic. Also
within the class of spiral galaxies there are trends: Only ~ 5% of the Sa
and Sab galaxies are peculiar at z ~ 0.7, while almost 70% of
the Sbc and Sc types are still peculiar.

Elmegreen et
al. (2007)
suggest that the
formation epoch of clumpy disk galaxies may extend up to z ~ 5.
The ones experiencing major mergers may form red spheroidals at 2
z 3,
whereas the others evolve into spirals.
Elmegreen et al. propose that the the star formation activity in
clumpy disks is caused by gravitational collapse of portions of the
disk gas without requiring an external trigger.

Regarding environment,
Poggianti et
al. (2009)
find that the fraction of ellipticals remains essentially constant
below z = 1, while the spiral and S0 fractions continue to evolve,
showing the most pronounced evolution in low-mass galaxy clusters.
They attribute this to secular evolution and to environmental
mechanisms that are more effective in low-mass environments. At low
redshifts, the declining spiral fraction with density is driven by
late-type spirals (Sc and later;
Poggianti et
al. 2008).

Irregular galaxies are gas-rich, low-mass, metal-poor galaxies without
spiral density waves, which show recent or ongoing star formation that
appears to have extended over a Hubble time
(Hunter 1997).
Many studies found the H I gas to be considerably more extended than the
stellar component in irregulars (e.g.,
Young & Lo 1997),
but more recent, deep optical surveys show that the optical extent of at
least some of these galaxies has been underestimated (e.g.,
Kniazev et
al. 2009).
All nearby irregulars and dwarf irregulars have
been found to contain old populations, although their fractions differ
(Grebel &
Gallagher 2004).
The old populations tend
to be more extended than the more recent star formation (e.g.,
Minniti &
Zijlstra 1996;
Kniazev et
al. 2009)
and show a more regular distribution (e.g.,
Zaritsky et
al. 2000;
van der Marel
2001).

Irregulars are usually found in the outskirts of groups and clusters
or in the field, thus interactions with other galaxies are likely to
be rare. Their star formation appears to be largely governed by
internal processes and seems to be stochastic. Rather than
experiencing brief, intense starbursts, irregulars typically show
extended episodes of star formation interrupted by short quiescent
periods - so-called gasping star formation (e.g.,
Cignoni & Tosi
2010).
The long-term star formation amplitude
variations amount to factors of 2 to 3
(Tosi et al. 1991).
For a review of irregulars and dwarf irregulars in
the Local Group, for which we have the most detailed data to date, see
Grebel (2004).

In contrast, in blue compact dwarf (BCD) galaxies the highest star
formation rates are found, and star formation occurs mainly in the
central regions (Hunter & Elmegreen). (We do not discuss BCDs and
other gas-rich dwarfs in more detail here. For an overview of
different dwarf types and their properties, see
Grebel 2003).

Once believed to be chemically homogeneous, there is now evidence of
metallicity variations at a given age in several irregulars (e.g.,
Kniazev et
al. 2005;
Glatt et al. 2008).
This suggests that local processes dominate the
enrichment and that mixing is not very efficient. Irregulars follow a
fairly well-defined metallicity-luminosity relation, which however is
offset from that of early-type dwarfs covering the same luminosity
range. Surprisingly, the offset is such that the continuously
star-forming irregulars and dwarf irregulars have lower metallicities
at a given luminosity than the inactive early-type dwarfs (e.g.,
Richer et al. 1998),
a discrepancy that holds even
when comparing stellar populations of the same age
(Grebel et
al. 2003).
Taken at face value, this may imply that
the enrichment of irregulars was less efficient and slower than that
of early-type dwarfs. BCDs and in particular extremely
metal-deficient galaxies continue this trend and appear to be too
luminous for their present-day, low metallicities even when compared
to normal irregulars
(Kunth & östlin
2000;
Kniazev et
al. 2003).

1. Galaxies with maximum rotational velocities Vmax
> 120 km s-1, total B-band magnitudes of MB -19, and stellar
masses
1010
M are mainly
bulge-dominated galaxies with relatively low specific star formation
rates and increased scatter in these rates. Also the mass-metallicity
relation changes its slope in this regime
(Panter et
al. 2007),
and supernova ejecta can be retained
(Dekel & Woo
2003).
Bothwell et
al. (2009)
find that the H I content of these massive galaxies decreases faster
than their star formation rates, leading to shorter H I
consumption time scales and making the lack of gas a plausible reason
for the observed quenching of star formation activity.

2. Galaxies with ~ 120 km s-1 > Vmax
> 50 km s-1 and -19 < MB < -15
comprise mainly late-type spirals and massive irregulars.
Lee et al. (2007)
suggest that spiral
structure acts as an important regulatory factor for star formation.
They find that the galaxies in this intermediate-mass regime exhibit a
comparatively tight, constant relation between star formation rate and
luminosity (or rotational velocity). The star formation rates show
fluctuations of 2 to 3, and the current star formation activity is
about half of its average value in the past.
Bothwell et
al. (2009)
argue that the galaxies in this regime evolve
secularly. They show that the star formation rates decrease
with the galaxies' H I mass, and that the H I
consumption time scales increase with decreasing luminosity.

3. Below Vmax = 50 km s-1 and
MB > -15, dwarf galaxies,
particularly irregulars, dominate. At these low masses, the star
formation rates exhibit much more variability ranging from
significantly higher (e.g., in BCDs) to significantly lower (e.g., in
so-called transition-type dwarfs with properties in between dwarf
irregulars and dwarf spheroidal galaxies, see
Grebel et
al. 2003)
star formation activity than in the higher-mass
regimes. Overall, there is a general trend towards lower star
formation rates. Stochastic intrinsic processes, feedback, and the
ability to retain gas play an important role here.
Bothwell et
al. (2009)
find that for many of the galaxies in the
low-mass regime the H I consumption time scale exceeds a Hubble
time (in good agreement with the results of
Hunter 1997).

Bothwell et al. show that the H I consumption time scales
have a minimum duration of more than 100 Myr. They argue that this
minimum duration corresponds to the gas mass divided by the minimum
gas assembly time, i.e., the free-fall collapse time.